1. Brief Description of the Invention
A method has been developed for the safe generation of metal carbonyl standards for the calibration of iron pentacarbonyl and nickel tetracarbonyl in CO by analytical methods such as FTIR, gas chromatography (GC), and mass spectroscopy (MS). Additionally these compounds can be used for the calibration of other direct introduction spectroscopic systems including but not limited to the sealed inductively coupled plasma (ICP), direct injection ICP-MS, and direct introduction ICP-AES.
2. Related Art
The literature describes two methods to produce pure carbonyls for commercial scale use.
A method for the production of iron carbonyl [Fe(CO)5] was described by Walls et al. (U.S. Pat. No. 2,378,053). They described a procedure for the production by passing CO over the metal of the carbonyl desired. The exact conditions of temperature and pressure were found to depend of the particular metal. Walls et al. described that the production of Nickel carbonyl [Ni(CO)4] proceeded easier than that for iron carbonyl. Reactions at atmospheric pressure and temperatures not to exceed 50xc2x0 C. could be used although it was common to use higher temperatures and pressures. It was also recognized that the reaction of Ni+CO and Fe+CO to produce the respective carbonyls proceeded by different kinetic factors. Even in the presence of S or S containing compounds, which were observed to increase the rate of reaction, these processes were deemed impractical for industrial applications by the authors.
British Patent 897,204 describes the preparation of chrome carbonyl [Cr(CO)6]. Previous to this patent chrome carbonyls had been formed by direct combination of CO with a salt of the metal in the presence of a strong reducing agent such as Li, Na or lithium aluminum hydroxide. The ""204 patent describes preparing the carbonyl from a Cr/Fe alloy which contained less than 40% Cr by weight in the presence of an aromatic sulphonic acid and an alkali metal hydroxide at temperatures between 200-300xc2x0 C. and pressures between 500-3000 atm. They noted that utilizing chrome alone (not in the presence of an alloy) did not give significant yields. The Cr/Fe alloy produced iron carbonyl and chrome carbonyl.
It has been observed by the present inventors that carbon monoxide (CO) filled in carbon steel vessels has the tendency to produce metal carbonyls, particularly iron carbonyl. There is a need in the semiconductor manufacturing art for determination of Fe and Ni as the carbonyl species in CO. Standards for calibrating laboratory equipment, such as FTIR, GC, ICP, ICP-MS, and ICP-AES, for the determination of metal carbonyl compounds, however are not commercially available. While it is possible to obtain pure iron pentacarbonyl, it is extremely toxic. Nickel tetracarbonyl, however is not stable and is not available as the pure source and is also extremely toxic. In order to prepare a ppm (v/v) standard of iron pentacarbonyl the liquid must be handled. It is not possible to prepare such a standard for nickel tetracarbonyl owing to the unavailability of the source starting material.
It would be a significant advance in the spectroscopic arts if a method were developed for the production of metal carbonyls, such as Fe and Ni carbonyls, at ppm levels which are stable for no less than 2 weeks.
In accordance with the present invention, methods of generating gaseous compositions comprising ppm levels of metal carbonyls are presented which overcome problems in the prior art.
One aspect of the invention is a method of preparing gaseous compositions comprising a metal carbonyl, preferably at ppm concentration, the method comprising the steps of:
(a) placing metal, preferably in the form of filings, of the metal carbonyl to be produced into a first test vessel at a first temperature;
(b) pressurizing the first test vessel with a gas comprising carbon monoxide from a carbon monoxide source vessel;
(c) heating the contents of the first test vessel to a second temperature and at a rate sufficient to initiate metal carbonyl formation, thereby forming a gas composition comprising a metal carbonyl;
(d) quenching the reaction of the carbon monoxide with the metal by transferring some of the gas composition comprising a metal carbonyl from the first test vessel to a second test vessel which is at a third temperature, the third temperature being lower than the second temperature; and
(e) diluting the gas composition comprising the metal carbonyl in the second test vessel with an inert gas (preferably argon, nitrogen, helium, or mixture thereof) from an inert gas source container.
It will be appreciated by those skilled in the art that more than one metal carbonyl can be produced simultaneously using the inventive method.
Preferred are methods in accordance with the invention wherein the first test vessel is a stainless steel test vessel, and the second test vessel is an aluminum test vessel.
Particularly preferred are methods in accordance with the invention wherein the metal is selected from the group consisting of Fe, Ni, Os, Ru, Ir, V, Mn, Cr, Co, Mo, and W, and wherein the metal carbonyl is selected from the group consisting of carbonyls of Fe, Ni, Os, Ru, Ir, V, Mn, Cr, Co, Mo, and W.
Preferably, the metal carbonyl is iron pentacarbonyl or nickel tetracarbonyl.
The physical form and amount of the metal influences the kinetics of the reaction of the metal with the carbon monoxide. Preferred are methods of the invention are those wherein the metal used to produced the carbonyl is in the form of a powder, having a mesh size ranging from about 100 to about 300 mesh, more preferably ranging from about 150 to about 250 mesh. Lower mesh sizes (higher average diameters of particles) tend to lower the rate of reaction, all other parameters being held constant, while higher mesh sizes (lower average particle diameters) tend to increase the reaction rate, due to surface area. The amount of metal present in the first test vessel also influences the reaction rate, with a higher amount present generally leading to higher production rate for the particular carbonyl of interest, although this has been found to be surprisingly not a linear relationship, as illustrated in the examples herein.
The temperature of the first test vessel, after the gas comprising carbon dioxide is introduced therein, is raised to at least 150xc2x0 C., more preferably at least 250xc2x0 C. The higher the temperature, and the greater the rise in temperature, the more metal carbonyl will be generated by the methods of the invention. The rate of increase of temperature is preferably at least 1xc2x0 C. per minute of heating, more preferably at least 10xc2x0 C. per minute of heating. The rate of heating does not have to be, but preferably is constant, and is preferably monitored with temperature indicating devices well known in the art.
The heating of the contents of the first test vessel may be accomplished by any means sufficient to do the job. Preferred methods include attaching a common electrical heating tape to the outside of the first test vessel, but other methods, such as radiation heating, immersion in an oven, or immersion in a heat transfer fluid, might have advantages in certain applications. A combination of these methods could also be contemplated.
Preferably the advancement and quenching of the metal carbonyl formation reaction are monitored by FTIR or other analytical technique.
The pressure in the first test vessel, which is essentially the carbon monoxide partial pressure, depends on the temperature to which the first test vessel is raised, but typically and preferably is at least 500 psig (34 atmospheres), more preferably at least 1000 psig (68 atmospheres). The pressure in the second test vessel is generally initially less than the pressure of the first test vessel the pressure, and is then raised by the addition of the inert gas. The final pressure of the gas composition comprising the metal carbonyl and the inert gas is typically and preferably above 1000 psig (34 atmospheres), but below about 2500 psig (170 atmospheres. As the metal carbonyls in general tend to be less stable at lower pressures, higher pressures are preferred. Of course this generally dictates more expensive test chambers.
A second aspect of the invention is a method of simultaneous production of nickel tetracarbonyl and iron pentacarbonyl, the method comprising the steps of:
(a) introducing iron and nickel into a first test vessel, preferably in the form of stainless steel powder;
(b) pressurizing the first test vessel with a gas comprising carbon monoxide from a carbon monoxide source vessel;
(c) heating the contents of the first test vessel to a second temperature and at a rate sufficient to initiate simultaneous nickel tetracarbonyl formation and iron pentacarbonyl formation, thereby forming a gas composition comprising nickel tetracarbonyl and iron pentacarbonyl;
(d) quenching the reaction of the carbon monoxide with the iron and nickel by transferring some of the gas composition comprising nickel tetracarbonyl and iron pentacarbonyl from the first test vessel to a second test vessel which is at a third temperature, the third temperature being at a temperature sufficient to significantly retard decomposition of the nickel tetracarbonyl and iron pentacabonyl (the third temperature preferably lower (preferably at room temperature, or about 20xc2x0 C.) than the second temperature); and
(e) diluting the gas composition comprising the nickel tetracarbonyl and iron pentacarbonyl in the second test vessel with an inert gas (preferably argon) from an inert gas source container.
A third aspect of the present invention are the gas compositions comprising metal carbonyls produced by the methods of the invention.
Further advantages and aspects of the invention will become apparent by reviewing the following description and claims.